The 0.618 method for grounding resistance measurement has long wire lead defect. By calculating and analyzing the groundpotential of a large number of grounding grids which are different in shape, area and rodarrangement style using computer numerical method, it presents that the grounding grids can be equivalent to disc shape grounding electrodes. A new method of grounding resistance measurement is presented that taking the currentelectrode wire as long as the longer side length of grounding grids and voltageelectrode wire as 0.509 times long as the currentelectrode wire. The measurement accuracy of this method is testified by experiment in field. The proposed method has much shorter wire than 0.618 method, it is suited to grounding resistance measurement of large grounding grids.